The majority of plastic materials for packaging are today based on petroleum. However the fossil resources on the earth are limited. Incineration results in an increase of the greenhouse effect and furthermore these materials are in general not degradable. A sustainable development in the future requires a conversion to the use of renewable raw materials.
In many food packaging applications it is important to protect the food from oxygen as oxidation of aroma compounds, due to the ingress of oxygen, reduces the quality and the flavour of the product. This can be done by using a barrier material, which has low permeability to oxygen. Furthermore, it is desirable that the material is flexible, mechanically resistant, transparent and of low cost.
EVOH (Ethylene vinyl alcohol) and PVOH (polyvinyl alcohol) are examples of synthetic polymers exhibiting good barrier properties.
Lately, research has been made to obtain oxygen barriers based on renewable raw materials. Films based on proteins or polysaccharides, such as starch and cellulose, have shown to be good barriers for oxygen. One drawback of these materials is their sensitivity to water. When the surrounding relative humidity is increased the oxygen permeability increases as well.
Hemicelluloses are polysaccharides that are biosynthesized in the majority of plants, where they act as a matrix material present between the cellulose microfibrils and as a linkage between lignin and cellulose. Hemicelluloses have been commercially used as sweetening agents, thickeners and emulsifiers in food. So far the non-food utilisation of hemicelluloses has been very limited. For example they have not yet been used commercially for the preparation of polymeric materials.
The properties of films based on hemicellulose have so far been very seldom studied. In general, hemicellulose exhibit poor film-forming properties resulting in either fragmented or very brittle films. However, the film-forming properties vary with the structure of the hemicellulose, which in turn is varying depending on its natural source and the extraction method. To be appropriate as a barrier material the film-forming properties have to be improved.
In WO 02/06411, the use of heteroxylans for the preparation of a film-forming composition containing a plant protectant is disclosed. The aim of WO 02/06411 is to provide a composition which is useful for applying a plant protectant to seeds or agricultural products. Thus, the purpose of incorporating heteroxylans is to obtain a film-forming composition for the application of the plant protectant.
The molecular weight of the heteroxylans used in WO 02/06411 ranges from 100 000 to 250 000 g/mol. The use of high molecular weight hemicellulose produces compositions having relatively high viscosities, which makes the compositions difficult to handle practically.
In U.S. Pat. No 6,004,616 a biodegradable film is obtained by subjecting water-soluble hemicellulose to film-formation. The hemicellulose used has an average molecular weight in the range of 50 000 to 1 000 000, preferably in the range of 100 000 to 400 000. Again, the high molecular weights present handling problems due to high viscosity.
Further, the thickness of the films described in U.S. Pat. No 6,004,616 is 0,1 mm in dried state. Thus, the films are relatively thick, which requires a lot of material to be consumed in the manufacture of the films. As a consequence thereof, the cost of materials will be very high.
There is thus a need for biodegradable film-forming compositions which overcome the abovementioned problems, and which presents the desired property of having low oxygen permeability.